Antennas may be used for a variety of purposes, such as communications or navigation, and portable radio devices may include broadcast receivers, pagers, or radio location devices (“ID tags”). The cellular telephone is an example of a portable communications device, which is nearly ubiquitous. Antennas for portable radios or wireless devices should be small, efficient, and have a broad radiation pattern.
Orientation of a portable device may be a concern. It may be impractical to orient a radio location tag, or point a cell phone, and satellites may tumble unintentionally. When antennas having radiation pattern nulls become misoriented, unacceptable fading is a common problem. Communications need to be reliable, and increased transmitter power may be required. Thus, a nondirectional antenna having a full-coverage radiation pattern may be desirable to avoid fading.
An example of a nondirectional antenna, which does not have radiation pattern nulls, is the isotropic antenna, which has a spherical radiation pattern for equal radiation in all directions. Isotropic antennas may provide a constant signal level for all antenna orientations, for operation without fading when the antenna cannot be aimed or pointed. The directivity of an isotropic antenna is 0.0 dB and if 100 percent efficient, the isotropic antenna gain is 0 dBi. Omnidirectional antennas may have circular antenna patterns in a single plane, such as for the horizon, and an isotropic antenna may provide omnidirectional patterns in all planes.
Antennas are transducers between electric currents and radio waves, and they may have a variety of shapes. Euclidian geometric shapes, such as those known through the ages, can be favorable for antennas. They can provide the greatest area for the perimeter (circles) or the shortest length between points (lines), etc. Thus, the two canonical antenna shapes may be the line and circle, corresponding to the dipole and loop type respectively.
The thin-wire half wave dipole is an example of a line shaped antenna. It may have a cos2 θ radiation pattern (two petal rose in plane) with two pattern nulls, a gain of 2.1 dBi, and a 3 dB gain bandwidth of 13%. Dipole antennas may be very common in the art, yet circle shaped antennas may have advantages for gain, polarization, and otherwise.
The full wave loop antenna is an example of a circle shaped antenna. It may have a circumference of 1 wavelength, a two petal rose radiation pattern (lobes broadside to the loop plane), and a gain of 3.6 dBi. U.S. Patent Application Publication No. 2008/0136720 to Parsche et al., assigned to the present assignee, and entitled “Multiple Polarization Loop Antenna and Associated Methods” discloses a full wave loop antenna with multiple feedpoints. Multiple polarizations may be provided from the single loop, including linear, circular, and dual polarizations.
A rectangular loop antenna was described by Heinrich Hertz in 1886. In his classic work, sparks were produced by radio, and the antenna was a 0.8×1.2 meter wire rectangle (“Electric Waves”, Heinrich Hertz, Macmillan 1893). Sparks were rendered at a gap in the antenna conductor, so the gap provided a detector and receiver. As the frequency neared 40 MHz, the loop was a half wavelength in perimeter, resonant (or “antiresonant”), and with a high impedance at the gap. While the high impedance was beneficial for high voltage sparks, high impedances may not be preferential for modern electronics since solid state devices operate at low voltages. For modern needs, a half wave circular loop antenna of a low driving impedance, for example, 50-Ohms may be desirable.
Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. Antennas become increasingly larger as the frequency decreases. At high frequencies (HF), 3 to 30 MHz for example, used for long-range communications, efficient antennas become too large to be portable, and wire antennas may be required at fixed stations. It becomes increasingly important in these communication applications to reduce not only the antenna size, but also to design and manufacture a reduced size antenna having the greatest gain for the smallest area.
U.S. Pat. No. 6,252,561 to Wu, et al. is directed to a wireless LAN antenna with a dielectric substrate having a first surface and a second surface. The first surface of the dielectric substrate has a rectangular loop. A rectangular grounding copper foil is adhered within the rectangular loop. A signal feeding copper foil is further included. One end of the signal feeding copper foil is connected to the rectangular loop and the grounding copper foil, while another end of the signal feeding copper foil runs across another end of the rectangular loop. Moreover, a layer of copper foil is plated to the back side of the printed circuit board. This back surface copper foil covers one half of the loop on the front surface. Adjustment of the transversal dimensions of the grounding copper foil will impedance-match the antenna to the feeding structure of the antenna.
Also, U.S. Pat. No. 6,590,541 to Schultze is directed to a half-loop antenna having an antenna half-loop positioned on top of a ground plane, the antenna half-loop forming an area whose outer edge forms a convex closed curve. The conductor half-loop has the form of an ellipse tapering to a point at its ends, and at the feed-in point of the conductor half-loop an inductance can be inserted, formed as a spring.
U.S. Pat. No. 4,185,289 to DeSantis et al. discloses a spherical body dipole including an annular slot feed. Complimentary radiation patterns provide near isotropic coverage. Yet, a smaller, planar radiating structure may be needed for portable personal communications, and a wire structure may be required for HF applications.
Prior approaches to forming isotropic antennas include optical approaches and or waveguides. U.S. Pat. No. 5,859,615 to Toland et al. is directed to an omnidirectional isotropic antenna using a tubular waveguide and an ellipsoid lens. U.S. Pat. No. 7,298,343 to Forster et al. is directed to an RFID tag that includes an antenna structure that is a hybrid loop-slot antenna.
However, none of these approaches are focused on providing an isotropic (radiates substantially equally in all directions) planar loop antenna component, e.g. for circuit boards, while being small in size, having desired gain for area, and with an adjustable feed impedance. Thus, there is a need for an easily manufactured, reduced size and cost, planar, isotropic loop antenna.